Lamps for vehicles such as automobiles have been proposed that use a semiconductor light-emitting element such as a light emitting diode (LED) as a semiconductor light source and use the LEDs for both a tail lamp and a stop lamp. (See, e.g., U.S. Pat. No. 6,873,111)
In order to use the LED for both the tail lamp and the stop lamp, a switching transistor is provided in series with the LED in an output loop of a circuit to control lighting of the LED. In addition, an on/off signal having a different duty ratio is applied to the switching transistor depending on whether operation is in a mode that makes the LED function as the tail lamp or a mode that makes the LED function as the stop lamp. Thus, the brightness of the LED is adjusted depending on the mode.
According to the foregoing patent document, the on/off signal having the different duty ratio is applied to the switching transistor depending on the mode so as to adjust the brightness of the LED. Therefore, the LED can be used for both the tail lamp and the stop lamp. The duty ratio of the on/off signal applied to the switching transistor changes according to the mode, and the peak value of the current supplied to the LED remains constant. Accordingly, when the light is reduced, such as when the LED functions as the tail lamp, the average current of the LED decreases according to the reduction in the duty ratio of the on/off signal. On the other hand, when the switching transistor is on, overshooting or undershooting occurs in the LED current.